31 



overlying" an impervious stratum which dips gently to the 

 south and probably to the west away from the face of the cliff. 



DIVERSIFIED SHORE AND CORAL REEFS. 



"The shore line of Oahu is much more irregular than the 

 shore line of any of the other islands. There are important 

 points on all sides of the island, the most prominent of which 

 are Diamond and Koko Heads, Makapuu, Mokapu, Kahuku, 

 Kaena and Barbers points. There are also good bays, the 

 most important of which are at Honolulu and Pearl harbors 

 on the south side. Pearl Lochs, 6 or 7 miles w^est of Honolulu, 

 is the site of the naval station and is said to form one of the 

 safest and best harbors on the Pacific. 



"Oahu has more coral on and around it than any of the 

 other islands. Extensive living coral reefs almost completely 

 girdle the island, closing the entrance to the bays except where 

 enough fresh water is received from streams or springs to 

 maintain an opening to the sea. The coastal plain which ex- 

 tends almost entirely around the island consists mainly of 

 uplifted coral, especially on the south side ; and coralline lime- 

 stone strata are encountered in well borings at various depths 

 below sea level. 



"The distribution of coral below and above sea level is one 

 of the principal evidences of long periods of subsidence fol- 

 lowed by later upheaval. Well borings show alternations of 

 basalt clay, earth, limestone, and hard basaltic sheets to a 

 depth of several hundred feet. Hard coral has been encoun- 

 tered at 800 feet below sea level, and broken coral at some- 

 what greater depth. These facts lead to the conclusion that 

 the island has been depressed 700 or 800 feet. Surface coral 

 near the shore indicated a later upheaval of 50 feet or more. 

 At \\"aipio, just west of Pearl Lochs, there is a stratum of oys- 

 ter shells 3 or 4 feet thick and_probably 20 feet or more above 

 sea level. 



HYDROGRAPHY. 



"The conditions just described have given to Oahu the dis- 

 tinction of having the best artesian water supply of any of the 

 islands. The principal water-bearing stratum is a vesiculated 

 basalt which lies 300 to 400 feet below sea level and which is 

 overlain by an impervious cover. The water in flowing wells 

 originally reached 42 feet above sea level at Honolulu. 32 feet 

 at Ewa. and 26 feet at Kahuku. The height is now consider- 

 ably less than it was originally. In addition to the large num- 

 ber of flowing- wells, there are many others which are pump- 

 ed. In all nearly 500 wells have been sunk on Oahu, chiefly 

 on the south side. 



"The rainfall on Oahu is comparatively less than on the 



